1. Field of the Invention
The present invention relates to a transposable element isolated from the genome of Ralstonia solanacearum and a transposase encoded by the transposable element.
2. Description of the Related Art
It is known that transposable elements not only inactivate or activate genes before and after the transposition site by moving on the genome, but also cause various genome rearrangements such as deletion, inversion and duplication. It is also known that the transposable elements contribute to genome plasticity that is important for the evolution and the environmental adaptation of microorganisms (Arber et al., FEMS Microb. Ecol., 15: 5-14 (1994)).
Transposable elements can be classified into transposons and insertion sequence elements. Transposons are defined as genes involved in transposition and having phenotypic genes such as antibiotic resistances. On the other hand, insertion sequence elements (hereinafter, referred to as IS elements) are defined as genes that generally have a size of 2 kb or less and have no phenotypic genes other than the genes involved in gene transposition (Gay et al., J. Bacteriol., 164: 918-921 (1985)).
The IS element was first discovered in the late 1960""s as an element that causes mutation of E. coli. Thereafter, the IS elements of E. coli in particular were isolated and characterized. In the late 1980""s, there were reports of isolation of the IS elements from bacteria other than the E. coli group. At present, about 500 IS elements are reported to have been isolated from medical bacteria, environmental bacteria and others (Mahillon et al., Microbiol. Mol. Bio. Rev., 62:725-774(1998)).
The isolated IS elements are compared with each other based on the base sequence and the deduced amino acid sequence. The IS elements that have been found so far are classified into 17 families. Among these families, the IS3 family and the IS5 family are large families, and IS elements belonging to these families are isolated from bacteria ranging widely from Gram-negative bacteria to Gram-positive bacteria (Mahillon et al., ibid.).
The structure of the IS elements in the IS3 and the IS5 families is characterized by the presence of inverted repeat sequences at their terminals and the inclusion of two open reading frames encoding a transposase. It seems that the terminal inverted repeat sequences play an important role in transposition, because recombination occurs in these portions. For example, in insertion, 2 to 13 base pairs in the site where insertion occurs are overlapped as same direct repeat sequences on both sides of the IS element (Galas and Chandler, Mobile DNA, 109-162, ASM Press (Washington, D.C.) 1989).
Transposase is an enzyme that catalyzes an insertion reaction of a gene. In the IS elements of the IS3 family and the IS5 family, the transposase is encoded by two open reading frames and is expressed as one protein in translation. For example, in the case of the IS3 family, two open reading frames overlap, and a frame shift occurs in the overlapping portion. Then, the two open reading frames are translated successively so that the transposase is expressed (Ohtsubo and Sekine, Transposable Elements, pages 1-26, Springer-Verlag Berlin Heidelberg, Germany (1996)).
Each of the IS elements obtained from various microorganisms has a sequence that is unique to the particular microorganism, although they are homologous. Utilizing the unique sequence, in the industrial fields of agriculture, foods, pharmacy and the like in which microorganisms are involved, the IS elements are used, for example, for identification of plant pathogenic bacteria, diagnosis of infectious diseases, determination of infection routes and isolation of useful genes. The IS element that is most common in practical use is IS6110, which is obtained from Mycobacterium tuberculosis and contributes to epidemiological surveys such as diagnosis of tuberculosis and determination of the infection route (Otal et al., J. Clin. Microbiol., 29:1252-1254 (1991)). Similar attempts have been conducted with various bacteria.
IS elements are characterized by being able to activate or inactivate genes before and after the inserted site by transposition. Utilizing this function positively, it is attempted to use the IS elements as a tool for isolating useful genes (Haas et al., Molecular Biology of Pseudomonas, 238-249, ASM Press (Washington D.C.) (1996)).
The isolation of the IS elements was not conducted strategically, but mostly was rather accidentally achieved by analysis of mutants. This is because, as seen from the definition, IS elements have no phenotype other than involvement in gene transposition, so that positive isolation was difficult. Various transposon trap vectors have been developed since positive selection of the transposable element (a method of utilizing activation of a detectable marker by transposition of the transposable element to a part of a structural gene) was successfully reported in 1985 (Gay et al., ibid.), and the IS elements have been isolated from bacteria such as Agrobacterium tumefaciens, Pseudomonas cepacia, Rhizobium meliloti, Rhizobium legumino sarum or the like by this method.
However, only 20 IS elements were isolated by this method, and this is a small number relative to the total number of the isolated IS elements. The isolation of new IS elements from various bacteria is expected to be accelerated as the positive selection is developed.
In regard to agriculturally important plant-related bacteria, Hasebe et al. isolated three kinds of IS elements from Pseudomonas glumae, which is a common pathogenic bacterium in rice plants in Japan, by utilizing a transposon trap vector pSHI1063 (Hasebe et al., Plasmid, 39:196-204 (1998)) (see Japanese Laid-Open Publication No. 10-248573). However, in reality, isolation of other IS elements has not been substantially researched. Isolation of IS elements of various plant pathogenic bacteria (microorganisms) makes it possible to counter the pathogenic bacteria, so that there is a demand for isolation and utilization of the IS elements in the field of agriculture.
Therefore, with the foregoing in mind, it is an object of the present invention to provide new IS elements derived from Ralstonia solanacearum, which is one of the common pathogenic bacteria in vegetables. The Ralstonia solanacearum is a worldwide common soil infectious plant pathogenic bacterium that causes wilt vessel disease to more than 100 species of more than 30 families including Solanaceae plants such as tomatoes, tobacco and potatoes. It is another object of the present invention to provide transposases that are encoded by these IS elements. These IS elements and the transposases make it possible to prevent infection to Ralstonia solanacearum, for example, by effecting recombination so that the transposases are inactivated, which is very useful in the field of agriculture.
In order to achieve the above objects, the inventors of the present invention have succeeded in isolating new IS elements from Ralstonia solanacearum and realized the present invention.
The present invention provides an insertion sequence element or a functional equivalent thereof comprising: a base sequence of Sequence I.D. No.2 at the 5xe2x80x2 terminal and a base sequence of Sequence I.D. No.3 at the 3xe2x80x2 terminal as terminal inverted repeat sequences; and a base sequence encoding amino acid sequences of Sequence I.D. Nos.4 and 5 as open reading frames between the terminal inverted repeat sequences. Herein, the open reading frame can be present overlapped or independently.
The present invention further provides an insertion sequence element consisting of the base sequence of Sequence I.D. No.1.
Furthermore, the present invention provides an insertion sequence element or a functional equivalent thereof comprising: a base sequence of Sequence I.D. No.7 at the 5xe2x80x2 terminal and a base sequence of Sequence I.D. No.8 at the 3xe2x80x2 terminal as terminal inverted repeat sequences; and a base sequence encoding amino acid sequences of Sequence I.D. Nos.9 and 10 as open reading frames between the terminal inverted repeat sequences. Herein, the open reading frame can be present overlapped or independently.
The present invention further provides an insertion sequence element consisting of a base sequence of Sequence I.D. No.6.
Furthermore, the present invention provides an insertion sequence element or a functional equivalent thereof comprising: a base sequence of Sequence I.D. No.12 at the 5xe2x80x2 terminal and a base sequence of Sequence I.D. No.13 at the 3xe2x80x2 terminal as terminal inverted repeat sequences; and a base sequence encoding an amino acid sequence of Sequence I.D. No.14 as an open reading frame between the terminal inverted repeat sequences.
The present invention further provides an insertion sequence element consisting of a base sequence of Sequence I.D. No.11.
The present invention further provides a transposase or a functional equivalent thereof expressed from a base sequence of positions 56 to 855 of Sequence I.D. No.1.
The present invention further provides a transposase or a functional equivalent thereof expressed from a base sequence of positions 65 to 822 of Sequence I.D. No.6.
The present invention further provides a transposase or a functional equivalent thereof expressed from a base sequence of positions 44 to 865 of Sequence I.D. No.11.
It should be noted that xe2x80x9cmovable genetic elementsxe2x80x9d (transposable elements) such as transposons may be significantly involved in the evolution and the environmental adaptation of organisms as self-mechanism of self-alternation of the organism""s genome. The gene of the present invention moves on the genome of microorganisms and has the nature of activating or inactivating the gene that is positioned downstream of the genome into which the gene of the present invention jumps. Utilizing this property, it is possible to isolate industrially useful genes efficiently. Furthermore, utilizing these genes, it is possible to determine the infection route and identify the bacteria in regard to Ralstonia solanacearum. Furthermore, the transposition function of the transposable element is promoted by utilizing the transposases of the present invention, so that it is possible to isolate industrially useful genes efficiently. In addition, it is expected that protectant effects such as promotion of a reduction in the pathogenicity caused by the mutation induction of Ralstonia solanacearum can be provided.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.